HELP!When images are chopped, how does the ALT tag work?

Hi! I'm just wondering, how does the ALT tag works if the images are chopped? I've read that some of the search engines use the image's ALT tag to index an image. What happens now if the image is chopped? What could be the possible solution to still index a particular image just by looking at ALT tag even the image is chopped? Thanks in advance to those who'll reply on this thread.

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